Monday, March 12, 2012

White Tank Mountains: Mesquite Canyon Trail Part 1

Saguaros along the Mesquite Canyon Trail.
The Mesquite Canyon Trail loop is the first hike into the mountains to be posted here and will introduce us to some of the common aspects of desert mountain ecology.  The White Tank Mountains are a small isolated range in the Sonoran Desert just west of Phoenix.  At the start of the hike at the mountain base the elevation is about 1,500 feet.  Peak elevation in the White Tanks is just above 4,000 feet but the peak elevation for this trail is around 3,000 feet along the Ford Canyon Trail portion of this loop.  The trail begins on the bajada which was deposited by Mesquite Canyon which the trail continues into.  This first section of trail through the canyon is quite steep as elevation increase about 500 feet in the first mile or so.  This is the roughest portion of the hike and after that it smooths out.  Mesquite Canyon Trail continues for a total of 3.1 miles until it reaches the Ford Canyon trail for which you take for 0.7 miles.  At that point you return to Mesquite Canyon via the Willow Canyon Trail which is 1.6 miles.  From there you return to the mid-point of the Mesquite Canyon Trail and continue downslope the way you came up through the canyon.  Total round trip is 8.1 miles. 

Starting at the ramada parking lot, the trail begins up a flat bajada gently sloping up towards the mountains.  This bajada was deposited by the canyon the trail continues into.  All of the sediments of this bajada were once carried by erosion off the mountain sides and by flashfloods through Mesquite Canyon and deposited outside the mountain to for an alluvial fan of the bajada.  Hiking into the canyon you can begin to see this bajada forming process.  The bajada begins as giant boulders from the mountain that are cracked and broken into increasingly small pieces until they form sand.  Most of the White Tanks are what is considered White Tanks Granite which forms a lot of sandy sediment when broken down.  Extreme desert heat, water, and plants all work together to break down the large rocks into sand.  As the rocks break down they are carried downslope and out of the canyon through the wash during flashfloods until they are deposited in an alluvial fan somewhere at the end of a wash.  Several alluvial fans deposited this way form a bajada.

At first in the canyon the trail continues on a north facing slope.  Looking closely, the plants are mostly Brittle Bush, Globe Mallow, and Jojoba, there is a peculiar absence of cacti.  Looking across the canyon to the south facing slope there are a lot more cacti and Palo Verde along with Brittle Bush.  Why the difference?  Both sides have the same granitic rocks and about the same soils so rocks or soil can’t explain the difference.  The south facing slope however, is far more exposed to the intense desert sun and therefore is significantly warmer and dryer than the north facing slope.  It probably is too cold for cacti to survive on the north facing slope and jojoba likely prefer the greater moisture content there.  North facing slope plants therefore likely can tolerate the cooler temperatures of that slope and are supported by the greater amount of moisture there.  South facing slope plants likely need the heat to survive and are able to tolerate the lack of moisture.  Also, looking closely at the soil you will notice there is an abundance of mosses, lichens, and other very low ground cover on the north facing slope.  This low ground cover is known as cryptobiotic soil crusts and again is a result of the greater amount of moisture on that slope.  The south facing slope with its lack of water due to greater sun exposure results in a near absence of cryptobiotic soil crusts.
Ferns that sprout from cryptobiotic soil crusts after rain.
Heading up the mountain, the trail climbs over a ridge top and continues along the side of a second canyon.  Here the trail levels off significantly and after a little ways the wash bed of this canyon can clearly be seen.  The wash bed is directly cut into the mountains granite.  It has been eroded and polished smooth by millenniums of sand washing over it and wearing it down.  The eroded smooth granite appears almost white in coloration and is full of small holes, known as tanks.  After rainfall and flash floods moving through the canyon these tanks fill with water.  On my recent hike all of the tanks were full of water due to rainfall a month prior.  These tanks, carved in whitish colored granite, are where the White Tank Mountains gain their name.  These tanks also are an amazing asset to the local wildlife being they hold water for months after rain falls.  Without these tanks there would be far fewer mule deer and mountain lions in the White Tanks. (Yes, it’s true, there is a healthy mountain lion population in the White Tanks.  So watch out!)

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